1. Field of the Invention
The present invention relates to the design of Ethernet passive optical networks. More specifically, the present invention relates to a method and apparatus for forwarding packets in an Ethernet passive optical network.
2. Related Art
In order to keep pace with increasing Internet traffic, optical fibers and associated optical transmission equipment have been widely deployed to substantially increase the capacity of backbone networks. However, this increase in the capacity of backbone networks has not been matched by a corresponding increase in the capacity of access networks. Even with broadband solutions, such as digital subscriber line (DSL) and cable modem (CM), the limited bandwidth offered by current access networks creates a severe bottleneck in delivering high bandwidth to end users.
Among the different technologies that are presently being developed, Ethernet passive optical networks (EPONs) are one of the best candidates for next-generation access networks. EPONs combine ubiquitous Ethernet technology with inexpensive passive optics. Hence, they offer the simplicity and scalability of Ethernet with the cost-efficiency and high capacity of passive optics. In particular, due to the high bandwidth of optical fibers, EPONs are capable of accommodating broadband voice, data, and video traffic simultaneously. Such integrated service is difficult to provide with DSL or CM technology. Furthermore, EPONs are more suitable for Internet Protocol (IP) traffic, because Ethernet frames can directly encapsulate native IP packets with different sizes, whereas ATM passive optical networks (APONs) use fixed-size ATM cells and consequently require packet fragmentation and reassembly.
Typically, EPONs are used in the “first mile” of the network, which provides connectivity between the service provider's central offices and business or residential subscribers. Logically, the first mile is a point-to-multipoint network, with a central office servicing a number of subscribers. A tree topology can be used in an EPON, wherein one fiber couples the central office to a passive optical splitter, which divides and distributes downstream optical signals to subscribers and combines upstream optical signals from subscribers (see FIG. 1).
Transmissions within an EPON are typically performed between an optical line terminal (OLT) and optical networks units (ONUs) (see FIG. 2). The OLT generally resides in the central office and couples the optical access network to a metro backbone, which is typically an external network belonging to an Internet Service Provider (ISP) or a local exchange carrier. An ONU can be located either at the curb or at an end-user location, and can provide broadband voice, data, and video services. ONUs are typically coupled to a one-by-N (1×N) passive optical coupler, where N is the number of ONUs, and the passive optical coupler is typically coupled to the OLT through a single optical link. (Note that one may use a number of cascaded optical splitters/couplers.) This configuration can significantly save the number of fibers and amount of hardware required by EPONs.
Communications within an EPON can be divided into downstream traffic (from OLT to ONUs) and upstream traffic (from ONUs to OLT). In the downstream direction, because of the broadcast nature of the 1×N passive optical coupler, downstream data frames are broadcast by the OLT to all ONUs and are selectively extracted by their destination ONUs. In the upstream direction, the ONUs need to share channel capacity and resources, because there is only one link coupling the passive optical coupler with the OLT.
To interoperate with other Ethernet equipment, an EPON needs to comply with the IEEE 802 standards, which specify two types of Ethernet operation: shared-medium operation and point-to-point operation. In a shared-medium Ethernet segment, all hosts are coupled to a single access domain over a common medium (e.g., a copper cable). Because the transmission medium is shared by all the hosts, only one host can transmit at a time while others are receiving. Point-to-point operation is proper when one link couples only two hosts. With a full-duplex point-to-point link, both hosts may transmit and receive simultaneously when communicating with each other.
An Ethernet bridge interconnects multiple Ethernet segments and forwards Ethernet frames among these segments. A bridge typically has a number of ports, each of which may be coupled to either a shared-medium segment or a point-to-point segment. According to the IEEE 802 standards, a bridge forwards a frame to a port associated with the frame's medium access control (MAC) destination address. A bridge does not forward a frame to a port on which it arrives. It is generally assumed that, if the frame's destination address is associated with the port on which the frame arrives, a frame's destination host is on the same shared-medium segment (called “broadcast domain”) as the source host. This is because communication between hosts on the same broadcast domain usually can be performed without the help of the bridge.
A bridge maintains a MAC address-port mapping table by associating the MAC source address of a frame with the port it arrives on. When an arriving frame's MAC destination address does not correspond to any port (i.e., the bridge has not established a MAC address-port mapping relationship for this address), the bridge floods the frame to every port except for the one on which the frame arrives.
In an EPON, an OLT generally behaves like an Ethernet bridge. The tree topology of an EPON, however, presents a problem: if the head end (OLT side) of the upstream link is coupled to one single port of the bridge residing in the OLT, the bridge will not forward any frames sent by an ONU to another ONU. This is because the entire EPON, which couples to the bridge through one port, appears to be a single shared-medium segment to the bridge. Because of the one-way broadcast nature of an EPON, an ONU cannot receive signals sent by other ONUs, unless the signals are switched and re-transmitted downstream. Fortunately, one can solve this problem by creating a logical link between each ONU and the OLT, and creating a virtual port on the bridge corresponding to this logical link. In this way, each ONU has its own logical port on the bridge, and operates as if there is a point-to-point link between the ONU and the OLT (this is called point-to-point emulation, PtPE). An upstream frame from an ONU is assigned a logical link identifier (LLID) that identifies to which virtual port this frame should go.
Although PtPE solves the bridging issue, the default bridge behavior of an OLT still has limitations. For example, the default flooding of a frame with unknown MAC destination address is not desirable. This is because some of the virtual ports are coupled to an upstream external network, and some are coupled to downstream ONUs. Consequently, frames arriving from a network-side virtual port may be flooded back to other network-side virtual ports. Such behavior is generally not desired for privacy purposes. Another limitation is that an OLT does not process information outside the Ethernet header, thereby prohibiting implementation of more intelligent switching functions.
Hence, what is needed is a method and an apparatus for forwarding packets in an EPON which allows more intelligent packet processing than an Ethernet bridge does.